Rising tennis star Ben Shelton said Friday that his career had moved "a lot faster than I would have thought" after advancing to the semifinals of the Japan Open.

The 21-year-old has quickly made a name for himself since turning professional in August last year, reaching the quarterfinals of the Australian Open in January and the semifinals of the US Open last month.

The world number 19 beat fellow American Tommy Paul 7-6 (7/4), 6-3 in Tokyo to move a step closer to his first ATP Tour title, and said he was "really happy with the way things are going".

"I didn't think right away that I was going to be a pro as soon as my college career was over, that I would be on the main ATP Tour this quickly," said Shelton, who will play another American, Marcos Giron, in the semifinals.

"Definitely things have moved a lot faster than I would have thought when I was in college."

Shelton said he is still taking classes at the University of Florida but would have to "see if it's something that I continue to do" or decide to leave his education until later.

He gave the Tokyo crowd another glimpse of his undoubted talent with an impressive win over Paul, the fifth seed who he also beat at the US Open.

Shelton said he had to play "more within myself" to deal with the strong wind blowing around the stadium.

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He was happy to continue his recent run of form after reaching the quarterfinals of last week's Shanghai Masters.

"Wins are tough out here on tour so to get three in a row is something that's a pretty cool feat for me, something that hasn't come often for me yet in my career so far," he said.

"The last three tournaments I've played, I got at least three wins so definitely really excited about the way things are going for me."

Shelton has also become known for celebrating wins with his now-trademark "dialled-in" gesture with an imaginary telephone.

He has yet to celebrate this week in Tokyo but he said he might reconsider "if I were to win the title here".

"I'm not really someone who thinks about celebrations before the match – it's kind of just a spontaneous thing," he said.

"I guess I haven't felt it as much this week."

Semifinal opponent Giron moved on after a 6-1, 6-4 win over Canadian number-eight seed Felix Auger-Aliassime.

World number 79 Giron came through qualifying and knocked out Norwegian number-two seed Casper Ruud in the second round.

The other semifinal will be between Russia's Aslan Karatsev and Japanese wild card Shintaro Mochizuki.

Mochizuki, ranked 215 in the world, knocked out top seed Taylor Fritz in the second round and he continued his run with a 7-5, 2-6, 7-5 win over Australia's Alexei Popyrin.

Karatsev beat number-four seed Alex de Minaur 6-3, 6-2 in the other quarterfinal, eliminating the last seeded player standing at the competition.

"You have to play until the end," said Karatsev.

"It doesn't matter how strong you hit the ball, you have to expect the ball coming back, so you have to finish not one time but 10 times."